The oxide film of In.sub.2 O.sub.3 -SnO.sub.2, called "ITO film, " exhibits the semiconductor characteristics of n-type conductivity. Its high conductivity plus visible light transmissivity make the ITO film useful in a wide range of applications. Recent applications include liquid-crystal display devices, thin-film electroluminescent display devices, radiation detector elements, transparent tablets of terminal equipments, defogging heater films for windowpanes, antistatic films, and selective permeable membranes for solar heat collectors. The ITO film is formed by a variety of known techniques, such as chemical processes, e.g., spraying which utilizes the pyrolysis of a compound, and chemical vapor deposition (CVD); and physical processes, e.g., vacuum evaporation and sputtering. Of these, sputtering is finding extensive adoption because of growing interest in its advantage that the production of films having large-area is permitted. Further films of low resistivity may be formed with good reproducibility.
In forming an ITO film by sputtering, a sputtering target consisting of indium oxide and tin oxide (hereinafter called "ITO target" for brevity) is used. ITO targets usually have been made by press-molding a powder mixture of indium oxide and tin oxide, with or without the addition of a dopant, at ordinary temperature and by then sintering the resulting green compact in an ambient air atmosphere at 1250.degree. to 1650.degree. C.
Unfortunately, indium oxide powder and tin oxide powder are difficult to sinter. The ITO targets made by the above process (cold-pressing and atmospheric sintering), therefore, have densities of only 4.2 to 5.0 g/cm.sup.3, or at most about 60 to 70% of the theoretical density value. Another factor placing a limit upon the density and hampering its rise to higher levels is the consideration of density reproducibility. It is generally accepted knowledge in the art is that if large targets are to be manufactured steadily on an industrial scale, it is necessary to keep their density from reaching above approximately 5.0 g/cm.sup.3. Disadvantages of these low density ITO targets so far pointed out have included:
a) Low sputtering rate (film forming rate) PA1 b) Short target life PA1 c) Pronounced surface blackening tendency of ITO, and PA1 d) Increased dust (particle) deposition on the substrate. PA1 (a) very high initial investment and difficulty of building large-size equipment, PA1 (b) high running cost due to the necessity of expensive mold and associated parts, and PA1 (c) lowered mass-production ability resulting from operation rates of at most one batch or two per day.
In view of these disadvantage, and for the enhancement of sputtering operation efficiency, and for the qualitative improvement of the resulting ITO film, there has been strong demand for ITO targets of higher densities.
An attempt has been made to apply hot pressing in the step of compacting the starting powder material in a series of, for the purpose of improving the density of ITO target. The compaction step is one in a series of the target-making steps. It was found possible in that way to increase the ITO target density with relative ease to about 6.7 g/cm.sup.3 (about 94% of the theoretical). Thus, hot pressing has come into use for the manufacture of high density (6.0 to 6.7 g/cm.sup.3) products.
However, the use of hot pressing in the manufacture of ITO targets has caused new problems, such as
Because of the adverse eventual effects of these difficulties upon the target price, the hot press technique can scarcely be regarded as a fully satisfactory means for commercial production.